1. Field of the Invention
The present invention relates to a method of refining unused carbon parts and used carbon parts which are used in the production of polycrystalline silicon.
Priority is claimed on Japanese Patent Application No. 2008-236178, filed Sep. 16, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
A production apparatus employing Siemens method is known as a polycrystalline silicon production apparatus. In this polycrystalline silicon production apparatus using the Siemens method, a number of silicon seed rods are arranged in the reactor. The silicon seed rods in the reactor are heated, and raw material gas including mixed gas of chlorosilane gas and hydrogen gas is supplied to the reactor to come into contact with the heated silicon seed rods. On a surface of a silicon seed rod, polycrystalline silicon is produced by a hydrogen reduction reaction and a thermal decomposition reaction of the raw material gas.
Electrodes or heaters made of carbon are used in this reactor so as not to contaminate the silicon. In the production of silicon for semiconductors, it is necessary to use particularly high-purity carbon parts. For this reason, carbon parts are refined prior to using them. When the carbon parts are refined, in a method shown in Japanese Patent Publication No. 2649166, a carbon part is placed in a flowing type reactor, and chlorine gas is flowed for 10 to 30 hours at 800 to 1100° C. to remove impurities inside the carbon parts. Additionally, the method shown in Japanese Examined Patent Application, Second Publication No. S39-12246 is a method of performing heat treatment continuously for at least 100 hours using exhaust gas at the production of polycrystalline silicon, thereby reducing a phosphorus component in carbon.
Additionally, a method shown in Japanese Examined Patent Application, Second Publication No. H1-40000 is a method of holding the temperature in a reactor, in which a carbon parts are placed, at 1000° C., and first performing heat treatment for about 3 hours in a gas including a halogen, for example, hydrogen chloride, then performing an evacuation to perform heat treatment for 4 hours in reduced pressure, and further performing heat treatment for 5 hours in a reduction atmosphere, such as a hydrogen atmosphere. Moreover, a method shown in Japanese Patent Unexamined Publication No. 2004-2085 is a method of performing a halogen treatment in a reactor, for example, for 20 hours at a high temperature of 2400 to 3000° C., and then, cooling carbon parts to 50° C. in an inert gas atmosphere, and taking out the carbon parts from the reactor, and a method shown in Japanese Patent Publication No. 3654418 is a method of peeling the silicon adhering to the surface of the carbon parts while removing a part of carbon base material, then performing heat treatment in chlorine gas atmosphere, and reducing impurities to reuse the carbon parts, when used carbon parts are recycled.
As described above, various kinds of methods have conventionally been suggested as the method of refining the carbon parts. However, impurities are contained into the inside of the carbon parts as well as the surfaces of the carbon parts. Therefore, in order to remove the impurities from the carbon parts, there is a problem in that a method of performing heat treatment on the carbon parts over a long time in the atmosphere of chlorine gas or the like at a high temperature is required, and the productivity is poor. Additionally, as for the method of reusing the carbon parts, since silicon adheres to the surfaces of the carbon parts in the production of polycrystalline silicon, removal of silicon and refinement of the carbon parts after the removal of silicon are required. Shaving off the silicon adhering to the surfaces of the carbon parts while removing a part of carbon member (base material) is shown in Japanese Patent Publication No. 3654418. However, polycrystalline silicon rods may collapse due to the deterioration of the strength of the carbon parts, which causes productivity decline.